1. Field of the Invention
Present invention relates to medical polymer materials sterilized by radiation and their use, particularly to medical polymer materials showing small deterioration or decomposition over time passage after being exposed to radiation such as γ ray or a microwave.
2. Description of the Related Art
There are many kinds of materials employed in medical use, while more functional materials are expected to be employed in medical field, notwithstanding; and metals, ceramics and polymers are pointed out as biomaterials. Required properties of the medical material include sterilization capability in product stage as important factor, in addition to desirable functions of medical material.
Sterilization is defined in Japan Pharmacopoeia as sterilizing or removing all microbes in materials. There are pointed out 12 sterilization methods of 5 groups in Japan Pharmacopoeia, among which, sterilization methods by high pressure vapor, ethylene oxide gas (EOG) and radiation are widely and generally employed for use in living body.
Sterilization condition of high pressure vapor is applied for about 20 minutes at 121° C. of 1.0 kg/cm2G of saturated vapor pressure in relative simple autoclave. Such condition is possible to be employed in sterilization of many kinds of medical materials that stand high pressure and temperature. However such sterilization methods are employed in almost no case of organic polymer materials, because of causing deterioration or decomposition, while many kinds of metals and ceramics are able to stand high pressure and temperature.
On the other hand, speaking to EOG steralization method, sterilization instruments are developed in order to improve effectiveness, efficiency and safety in sterilization, because EOG is known as effective as a disinfectant. This sterilization method seems best in comparison with others in order to prevent deterioration of the raw materials. However EOG sterilization method recently come to be avoided in employing, because it is pointed out that EOG remains in medical materials after sterilization, and has bad effect on living body. According to recent research, sterilization method employed in suture for operation, disposable surgical gown and a hypodermic syringe, comes to be changed from usual EOG to radiation sterilization methods, because of high toxicity of residual EOG and complicated procedure for packing.
Radiation sterilization method is taken attention as other sterilization method, in which used are radial rays that are such beam of particles inclusive of light, coming from radioactive disintegration or the like, including α, β, γ rays, and neutron and X ray. Generally applied for the sterilization of medical polymer material are known to be γ ray coming from 60Co and electron beams.
As γ and electron beams have no serious damage to general-purpose polymer materials such as polyethylene, polypropylene, polyethylene chloride, polystyrene and synthetic rubbers, those polymers are possible to be sterilized by radiation and have been widely used.
However, it is pointed out that coloration, bad smells and oxidation degradation, happen to occur on passage of time, and are caused on generally employed sterilization irradiation amount of about 25 kGy. Thus, new sterilization methods such as plasma and ultra-violet methods are developing. The plasma and ultra-violet methods are partly applied as a simple and easy method, but has a very narrow application because of permeability.
Radiation exposure effects on the polymer materials are distinguished by cross-linking and collapse of the polymer chains, which are independent on irradiation rate, but proportional to irradiated dose over wide range. Cross-linking is defined as follows; radicals are generated by irradiation process, in which new inter- and intramolecular connections are generated on polymer molecules, molecular weight of the polymer finally increases to infinity, and the cross-linked polymer is insoluble in all kinds of solvents. On the other hand, the collapse of the polymer is defined as follows; polymers happen to deteriorate their molecular weight and mechanical properties, because the polymer chains are cut by irradiation.
The polymers are classified into cross-linking and collapsing type polymers dependent on radiation effect. Typical examples of cross-linking polymer are polyethylene, polypropylene, nylon, polystyrene, polyester, natural rubber and silicon resin. On the other hand, collapsing type polymers include poly-methylmethacrylate, Teflon (registered trademark), poly-isobutylene, collagen, cellulose and bio-decomposable polymers such as poly-lactic acid. However, even cross-linking type polymers happen inevitably to deteriorate in mechanical properties similar to the collapsing type ones because the polymer chains are cut by irradiation and generated radicals. Therefore, even polyethylene, which is easiest in cross-linking by radiation, happens to deteriorate in mechanical properties by time passage after irradiation. It is because radicals generated by irradiation do not extinct in short time, but remain in the polymer material for long time as free radicals and cause cutting of polymer chain by reacting with oxygen by passage of time.
The polymers are heat-treated at a temperature higher than 100° C. in order to remove the free radicals. However, when the polymers are heat-treated for several hours at a temperature higher than 100° C. in order to prepare cup and plate for artificial hip and knee joints made from ultra high molecular weight poly ethylene (UHMWPE), there is fear of deforming of the product by remained strain.
Many polymer materials made of polypropylene and sterilized by radiation are available for disposable medical materials such as suture for operation, hypodermic syringe, filter, surgical gown, and non-woven sheet. It is proposed the methods of compounding the propylene with hydrated rosin-methyl-ester (JP61-213243A) or styrene resin (JP-07-157922A) in order to prevent deterioration and bad smell arisen from sterilizing by radiation. However, it is not quite effective because the free radicals cannot completely be removed from by the above additives.
Only several kinds of polymers including polypropylene and polyethylene are suitable for sterilizing by radiation among many cross-linking types of decomposable and bio-absorbable polymers. Reason why radiation sterilization method is not employed in even radiation cross-linking type polymers of nylon or poly-vinylidene fluoride, is because the polymers happen to partly cross-link each other and partly decompose at same time, generate oligomers and monomers, and may cause toxicity. Furthermore, we cannot employ the radiation sterilization method on the suture for surgical operation, because the suture requires mechanical properties as maintained in long time and because of toxicity problem. Recently, notwithstanding that poly methyl methacrylate and silicon resin are generally employed on contact lenses and ocular lenses in accordance with their excellent optical properties and biological compatibility, EOG sterilization is inevitable in end product because radiation is hazardous for these polymers. It is problem that EOG remains in medical materials after sterilization. As eye is the most sensitive organ in comparison with other organs and tissues, it is all the more worrisome to give bad effect on living body.
On the other hand, medical polymer materials for implantation recently include suture for operation made from decomposable and bio-absorbable polymer, artificial dura mater, bonding agent for broken bone; and it is expected to grow in various uses. In spite of a fact that it passed more than 30 years since the suture for operation made from decomposable and bio-absorbable polymer was applied in clinic, EOG sterilization method is still employed. It is because the decomposable and bio-absorbable polymer is by nature composed of chemical structure that is unstable in heat, light, radiation and moisture.
Sterilization methods by high pressure vapor of EOG and radiation are widely and generally employed as mentioned above. Targets of present invention are to solve the following problems of radiation sterization method.                (1) Prevention of medical material deterioration caused on irradiation.        (2) Expansion of way of use of radiation steralization method to medical materials that have been impossible to be employed in past.        
Moreover, if it is possible to sterilize non-decomposable and non-bio-absorbable polymer by radiation, advantageous effect is immense. Especially, if it is possible to apply radiation sterilization method for decomposable and bio absorbable polymer for medical use that have been impossible to be applied with radiation sterilization in past, we cannot guess the usefulness in social and economical scopes.